The present invention relates to a process for electrochemically removing high spots from the surface of reactive metal work pieces. It utilizes parallel-to-face, as opposed to perpendicular-to-face, electrochemical machining with a shaped electrode.
Typically, electrochemical machining is done in a perpendicular-to-face (cavity sinking) mode. Generally, electrolyte is pumped through the center of the electrode and flows outward, typically in an uncontrolled manner off the workpiece and into a sump from where it is recirculated. A voltage between the tool and the workpiece allows material to be deplated off the workpiece surface through the electrically conductive electrolyte (e.g. NaCl or NaKNO.sub.3. Generally, the tool is fed into the workpiece at a conservatively slow speed to avoid arcing.
Avoiding arcing is especially crucial in reactive metals as arcing can create large particles of insoluble oxide which interfere with surface removal. As a result, electrochemical machining is generally not used with reactive metal workpieces.
Reactive metal parts have generally been finished by grinding. Grinding, however, is a slow and expensive process.
In U.S. Pat. No. 3,445,372, issued to Fromson on May 20, 1969, electrochemical machining parallel to the surface is taught using shallow angles down to zero, and an electrode shaped to uniformly remove material by compensating for circular travel. All portions of the workpiece are under the electrode for the same length of time, the electrode being larger moving out from the center of rotation to compensate for the more rapidly moving surface. In the preferred embodiment with an angled face, speed of travel is dictated by the angle and rate of material removal.
It has been discovered that reactive metal workpieces can be electrochemically machined to remove high spots. This can be done successfully on both sheet and tubes. It has been found that in tubes, for example, such imperfections generally taper relatively smoothly in all directions from a high spot. Thus, a single wedge-shaped tool can be used to remove most imperfections from a given type of product.
The speed and/or current can be controlled to vary the amount of material removed along center line of tool movement and the wedge shape of the tool results in correspondingly less material removal off the tool center line (e.g. with a triangular tool material removal with respect to center line removal is inversely proportional to distance from center line).
First, the workpiece is measured to locate a high spot to be removed. Then, a wedge-shaped, zero-angle electrochemical machining tool is positioned such that the region where the most metal will be removed is in line with the center of the tool. The tool is then moved parallel to the workpiece surface across the high spot at an inverse of speed times current product essentially proportional to the amount of material to be removed along the tool movement center line. Thus, as the tool is moved, initially relatively little material is removed, then as it approaches the high spot more and more material is removed, reaching a maximum removal at the high spot and then tapering back down. With sheet, removal of center thickening (such as results from roll deflection in a rolling mill) may be relatively constant and constant speed and current may be used for relatively long distances, with the wedge shape of the tool providing for less material removal off the center line of the strip.